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IGCSE Chemistry · 0620

IGCSE Chemistry: Stoichiometry and Mole Calculations

Mole calculations are where the most marks are won and lost in IGCSE Chemistry. These worked questions cover moles from mass, reacting masses from balanced equations and concentration in mol per dm3.

What you need to know

  • Number of moles equals mass in grams divided by the relative formula mass (Mr).
  • The balanced equation gives the mole ratio between reactants and products.
  • Concentration in mol per dm3 equals moles divided by volume in dm3; 1000 cm3 equals 1 dm3.
  • Always balance the equation first, then work in moles, then convert back to mass or volume.

Practice questions with answers

Question 12 marks

Calculate the number of moles in 8 g of sodium hydroxide, NaOH. (Mr of NaOH = 40)

Moles equals mass divided by Mr. Moles = 8 divided by 40 = 0.2 mol.

Mark schemeCorrect method, mass over Mr (1). Correct answer 0.2 mol (1).

Common mistakeMultiplying mass by Mr instead of dividing.

Exam tipWrite the formula moles equals mass over Mr before substituting. It earns the method mark even if the arithmetic slips.

Question 24 marks

Magnesium burns in oxygen: 2Mg + O2 gives 2MgO. Calculate the mass of magnesium oxide formed from 2.4 g of magnesium. (Ar: Mg = 24, O = 16)

Moles of Mg = 2.4 divided by 24 = 0.1 mol. From the equation, the mole ratio of Mg to MgO is 2 to 2, which is 1 to 1, so moles of MgO = 0.1 mol. Mr of MgO = 24 plus 16 = 40. Mass of MgO = 0.1 times 40 = 4 g.

Mark schemeMoles of Mg = 0.1 (1). Mole ratio 1 to 1, so 0.1 mol MgO (1). Mr of MgO = 40 (1). Mass = 4 g (1).

Common mistakeForgetting to use the mole ratio from the balanced equation.

Exam tipLay the calculation out in clear steps: moles, ratio, Mr, mass. Each step can earn a mark.

Question 32 marks

Calculate the concentration, in mol per dm3, of a solution containing 0.5 mol of solute in 250 cm3.

Convert the volume: 250 cm3 = 0.25 dm3. Concentration = moles divided by volume = 0.5 divided by 0.25 = 2 mol per dm3.

Mark schemeConvert 250 cm3 to 0.25 dm3 (1). Concentration = 2 mol per dm3 (1).

Common mistakeForgetting to convert cm3 to dm3, giving an answer 1000 times too small.

Exam tipDivide any volume in cm3 by 1000 before using it in a concentration calculation.

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Frequently asked questions

What is the formula for moles?

Number of moles equals mass in grams divided by the relative formula mass, Mr. Rearranged, mass equals moles times Mr.

What is Avogadro's number?

One mole of any substance contains 6.02 times 10 to the power 23 particles. This is Avogadro's constant.

How do you convert cm3 to dm3?

Divide the volume in cm3 by 1000. For example, 250 cm3 equals 0.25 dm3.

Why must the equation be balanced first?

The balanced equation gives the mole ratio between substances. Without it you cannot link the moles of one substance to another.

More IGCSE Chemistry practice

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